Vacuum booster apparatus and a brake apparatus

ABSTRACT

A vacuum booster apparatus used for a brake apparatus positively decreases a degree of opening of a throttle valve when it is required while an unnecessary decrease in the degree of opening of the throttle valve is positively prevented. A first pressure chamber of a vacuum booster is connected to an intake pipe of the engine on a downstream side of a throttle valve. A second pressure chamber of the vacuum booster is selectively connectable to one of the first pressure chamber and atmosphere. The vacuum booster boosts an operating force applied to a brake operating member based on a pressure difference between the first pressure chamber and the second pressure chamber. A throttle-opening-degree control device controls a negative pressure in the first pressure chamber by controlling the degree of opening of the throttle valve. The throttle-opening-degree control device decreases the degree of opening of the throttle valve only for a limited time when the negative pressure in the first pressure chamber is smaller than a predetermined necessary negative pressure determined by an operating-force relating amount related to the operating force applied to the brake operating member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vacuum booster apparatus and a brakeapparatus including a vacuum booster.

2. Description of the Related Art

Japanese Laid-Open Patent Application No. 8-164840 discloses a vacuumbooster apparatus provided with a vacuum booster. The vacuum boostercomprises a first pressure chamber which is connected to an intake lineof an engine at a position downstream of a throttle valve and a secondpressure chamber which is selectively connectable to either the firstpressure chamber or atmosphere. The vacuum booster boosts an operationalforce applied to a brake-operating member based on a difference betweena pressure in the first pressure chamber and a pressure in the secondpressure chamber. The vacuum booster apparatus also includes athrottle-opening-degree control apparatus which increases a negativepressure in the first pressure chamber by decreasing a degree of openingof the throttle valve. In the above-mentioned vacuum booster apparatus,when the negative pressure in the first pressure chamber becomes lessthan a predetermined negative pressure, the degree of opening of thethrottle valve is decreased so as to maintain the negative pressure inthe first pressure chamber at a pressure higher than the predeterminednegative pressure. Accordingly, since the negative pressure in the firstpressure chamber is maintained higher than the predetermined negativepressure whether or not the brake-operating member is being operated, aboost limit is prevented from being decreased. The boost limit isrepresented by a master cylinder pressure at a time when a boost actionreaches a limit.

The above-mentioned engine having the throttle valve is adirect-injection-type gasoline engine in which gasoline is directlyinjected into cylinders. In this type of engine, a uniform combustion isperformed when a load applied to the engine is large, that is, when anengine speed is high. On the other hand, when a load applied to theengine is not very large, that is, when the engine speed is medium orlow, a stratified charge combustion is performed. In the uniformcombustion, a combustion is achieved at a normal air-fuel ratio, thatis, a stoichiometric air-fuel ratio. In the stratified chargecombustion, an ultra lean burn occurs at a high air-fuel ratio of about25 to 50. According to the ultra lean burn, a rate of fuel consumptionis reduced which results in energy saving. When the engine load ismedium or low, a necessary drive torque can be output even when theultra lean burn occurs. Since, normally, the engine load is not verylarge when the brake operating member is being operated, the ultra leanburn is performed. In such the case, a degree of opening of the throttlevalve is maintained at a relatively large degree so as to intake a largeamount of air.

If the degree of opening of the throttle valve is maintained at arelatively large degree, the negative pressure in the first pressurechamber tends to be decreased. Accordingly, in the conventional vacuumbooster apparatus, the degree of opening of the throttle valve isdecreased when the negative pressure in the first pressure chamber isless than the predetermined negative pressure. When the degree ofopening of the throttle valve is decreased, a state of combustion shouldbe changed from the ultra lean burn to the uniform combustion. This isbecause an unstable combustion causing misfire occurs when the degree ofopening of the throttle valve is decreased. Additionally, the changefrom a state of the ultra lean burn to a state of the uniform combustionis performed stepwisely (gradually). If an output torque is the same, anamount of injection of fuel, the degree of opening of the throttle valveand a degree of opening of a swirl port are considerably differentbetween the state of the ultra lean burn and the state of the uniformcombustion. Accordingly, when the state of the ultra lean burn ischanged to the uniform combustion, these conditions are considerablychanged, which results in deterioration of drivability. Similarly, thereturn from the state of the uniform combustion to the state of theultra lean burn is performed stepwisely. As mentioned above, when thedegree of opening of the throttle valve is decreased when the ultra leanburn is being performed, it is required to stepwisely or graduallychange the ultra lean burn to the uniform combustion. This causesdeterioration in a rate of fuel consumption. Accordingly, it is notpreferable to decrease the degree of opening of the throttle valve so asto maintain a negative pressure in the first pressure chamber in a statein which the ultra lean burn is performed. Additionally, a frequency ofsuch a change in the state of combustion should be as small as possible.On the other hand, when a braking operation is not being performed,there is a possibility that the uniform combustion is performed. In sucha case, the rate of fuel consumption is not decreased by much even ifthe degree of opening of the throttle valve is decreased.

If the predetermined negative pressure at which the degree of opening ofthe throttle valve is decreased can be decreased, that is, if thepredetermined negative pressure can be set closer to an atmosphericpressure, a frequency of the degree of opening of the throttle valvebeing decreased can be decreased. However, in such a case, the limit ofboost is decreased. On the other hand, if the predetermined negativepressure is increased, that is, if the predetermined negative pressureis set closer to an absolute vacuum, the limit of boost can beincreased. That is, the boosting function can be provided until anoperational force applied to the brake-operating member is large.However, in such a case, the frequency of a degree of opening of thethrottle valve being decreased is increased. Additionally, since thevacuum booster cannot reach the limit of boost even if the negativepressure is less than the predetermined negative pressure in a rangewhere an operational force applied to the brake-operating member issmall, the degree of opening of the throttle valve is decreased despitethat the negative pressure is not required to be increased. As discussedabove, in the above-mentioned vacuum booster apparatus, it is difficultto decrease a frequency of the degree of opening of the throttle valvebeing decreased while a necessary negative pressure is maintained in thefirst pressure chamber.

In the above-mentioned patent document, a vacuum booster apparatus whichcan decrease the frequency of the degree of opening of the throttlevalve being decreased during a braking operation is also disclosed. Thisvacuum booster apparatus includes the above-mentioned vacuum booster anda throttle-valve-opening control apparatus. The throttle-valve-openingcontrol apparatus increases a negative pressure in the first pressurechamber by decreasing the degree of opening of the throttle valve whenthe brake-operating member is continuously operated for a period longerthan a predetermined period and when the negative pressure in the firstpressure chamber is less than the predetermined negative pressure. Inthis vacuum booster apparatus, the degree of opening of the throttlevalve is always decreased when the brake-operating member iscontinuously operated for a period longer than the predetermined periodand when the negative pressure in the first pressure chamber is lessthan the predetermined negative pressure. However, when an operationalforce applied to the brake-operating member is small, the vacuum boosterdoes not reach the limit of boost even if the braking operationcontinues for a long time and the negative pressure is small. In such acase, the negative pressure is not required to be increased.Additionally, when the operational force is large, the vacuum boostermay reach the limit of boost even if the braking operation continues foronly a short time. In such a case, the negative pressure should beincreased. As discussed above, in the above-mentioned vacuum booster,the degree of opening of the throttle valve may be unnecessarilydecreased or not decreased when it is required.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a vacuumbooster apparatus and a brake apparatus in which the above-mentionedproblems are eliminated.

A more specific object of the present invention is to provide a vacuumbooster apparatus and a brake apparatus which positively decreases adegree of opening of a throttle valve when it is required while anunnecessary decrease in the degree of opening of the throttle valve ispositively prevented.

In order to achieve the above-mentioned objects, there is providedaccording to one aspect of the present invention a vacuum boosterapparatus adapted to be used for a brake apparatus provided in a vehiclehaving an internal combustion engine, the vacuum booster apparatuscomprising:

a vacuum booster having a first pressure chamber and a second pressurechamber, the first pressure chamber being connected to an intake pipe ofthe engine on a downstream side of a throttle valve, the second pressurechamber being selectively connectable to one of the first pressurechamber and atmosphere, the vacuum booster boosting an operating forceapplied to a brake operating member based on a pressure differencebetween the first pressure chamber and the second pressure chamber; and

a throttle-opening-degree control device controlling a negative pressurein the first pressure chamber by controlling a degree of opening of thethrottle valve,

wherein the throttle-opening-degree control device includes limited-timeopening-degree decreasing means for decreasing the degree of opening ofthe throttle valve only for a limited time when the negative pressure inthe first pressure chamber is smaller than a predetermined necessarynegative pressure determined by an operating force relating amountrelated to the operating-force applied to the brake operating member.

According to the present invention, the operating force relating amountis related to the operating force applied to the brake operating membersuch as a brake pedal. That is, the operating force relating amountincludes the operating force itself and an amount in which the operatingforce is reflected such as a travel of the brake operating member or amaster cylinder pressure.

In the vacuum booster apparatus according to the present invention, thepredetermined necessary negative pressure is determined in accordancewith the operating force relating amount that relates to the operatingforce applied to the brake operating member. That is, the predeterminednecessary negative pressure is stepwisely or gradually increased as theoperating-force relating amount is increased. Additionally, the degreeof opening of the throttle valve is decreased only when the negativepressure in the first pressure chamber is smaller than the predeterminednecessary negative pressure.

The vacuum booster is closer to its boost limit when the operating-forcerelating amount is large than when the operating-force relating amountis small. Accordingly, when the operating-force relating amount islarge, the negative pressure in the first pressure chamber is preferablyincreased so as to increase a boost limit of the vacuum booster so thatthe vacuum booster is prevented from reaching its boost limit. On theother hand, when the operating force relating amount is small, there isno need to increase the boost limit. That is, it is not required toincrease the negative pressure in the first pressure chamber. Accordingto the vacuum booster apparatus of the present invention, since thepredetermined necessary negative pressure is determined in accordancewith the operating-force relating amount, the degree of opening of thethrottle valve can be decreased only when it is necessary.

In the vacuum booster apparatus according to the present invention, thelimited-time opening-degree decreasing means may includenecessary-negative-pressure determining means for determining thepredetermined necessary negative pressure in accordance with anoperating-force relating amount related to the operating force appliedto the brake operating member.

The predetermined necessary negative pressure may be determined by theoperating-force relating amount alone or with other conditions of abraking operation such as a period of operating the brake operatingmember and a rate of increase in the operating-force relating amount.

Additionally, in the vacuum booster apparatus according to the presentinvention, the limited-time opening degree decreasing means may includemeans for determining the predetermined necessary negative pressure inaccordance with a rate of increase in an operating-force relating amountrelated to the operating force applied to the brake operating member.

According to this invention, the predetermined necessary negativepressure is determined according to the operating force relating amountand the rate of increase in the operating-force relating amount. Thatis, the predetermined necessary negative pressure is increased as therate of increase is increased. Thus, if the operating-force relatingamount is the same, the predetermined necessary negative pressure whenthe rate of increase in the operating force relating amount is large isset to be greater than when the rate of increase in the operating-forcerelating amount is small.

When the rate of increase in the operating-force relating amount islarge, a rate of evacuation of air in the first pressure chamber ispreferably increased so as to maintain an appropriate operational speedof the vacuum booster. On the other hand, when the rate of increase inthe operating-force relating amount is small, there is no need toincrease the predetermined necessary negative pressure. Accordingly,when the operating-force relating amount is the same, the predeterminednecessary negative pressure when the rate of increase in theoperating-force relating amount is large is preferably set to be greaterthan when the rate of increase in the operating force relating amount issmall.

Additionally, the rate of increase in the operating force relatingamount is set to be a large value when it is required to rapidlydecrease a speed of a vehicle. In such a case, a possibility for thevacuum booster reaching it boost limit is high. Thus, it is preferableto increase the negative pressure in the first pressure chamber. On theother hand, when the rate of increase in the operating-force relatingamount is small, the possibility for the vacuum booster reaching itsboost limit is low. In this case, there is no problem if the negativepressure in the first pressure chamber is small. Particularly, when anoperating force applied to the brake operating member is released, therate of increase in the operating-force relating amount becomes anegative value. In this case, the possibility for the vacuum boosterreaching its boost limit is low. That is, a margin to the boost limitwhen the rate of increase in the operating-force relating amount islarge is smaller than when the rate of increase in the operating-forcerelating amount is small.

Additionally, in the vacuum booster apparatus according to the presentinvention, the limited-time opening degree decreasing means may decreasethe degree of opening of the throttle valve only when a boost limit ofthe vacuum booster is smaller than an operational state value which isdetermined according to an operating-force relating amount related tothe operating force applied to the brake operating member.

The boost limit of the vacuum booster is equal to the master cylinderpressure at a time when a boost action of the vacuum booster reaches itslimit. Accordingly, the operational state value can be the mastercylinder pressure as the operating force relating amount at that time.Additionally, the operational state value can be set as other valuessuch as the operating-force applied to the brake operating membercorresponding to the master cylinder pressure or a travel of the brakeoperating member. Further, the operational state value can be a valuedetermined by the operating-force relating amount and the rate ofincrease in the operating force relating amount.

The boost limit of the vacuum booster is determined by a pressuredifference between the first pressure chamber and the second pressurechamber when the pressure in the second pressure chamber reaches anatmospheric pressure. The boost limit is increased as the pressuredifference is increased. The pressure difference is increased as thenegative pressure in the first pressure chamber is increased.Accordingly, the boost limit is increased as the negative pressure inthe first pressure chamber is increased. A level of the negativepressure in the first pressure chamber needed for increasing the boostlimit to be greater than the operational state value is determined by alevel of the operational state value, and such a negative pressure isreferred to as a boosting-time necessary minimum negative pressure. Ifthe degree of opening of the throttle valve is decreased only when theboost limit is smaller than the operational state value, the negativepressure in the first pressure chamber can be maintained to be greaterthan the boosting-time necessary minimum negative pressure. Thus,measures should be taken for a decrease in the degree of opening of thethrottle valve only when the negative pressure in the first pressurechamber is smaller than the boosting-time necessary minimum negativepressure.

Additionally, there is provided according to another aspect of thepresent invention a vacuum booster apparatus adapted to be used for abrake apparatus provided in a vehicle having an internal combustionengine, the vacuum booster apparatus comprising:

a vacuum booster having a first pressure chamber and a second pressurechamber, the first pressure chamber being connected to an intake pipe ofthe engine on a downstream side of a throttle valve, the second pressurechamber being selectively connectable to one of the first pressurechamber and atmosphere, the vacuum booster boosting an operating forceapplied to a brake operating member based on a pressure differencebetween the first pressure chamber and the second pressure chamber; and

a throttle-opening-degree control device controlling a degree of openingof the throttle valve in accordance with a negative pressure in thefirst pressure chamber and at least one of an operating-force relatingamount related to the operating force applied to the brake operatingmember and a rate of increase in the operating-force relating amount.

In the vacuum booster apparatus according to this invention, the degreeof the throttle valve is controlled based on the negative pressure inthe first chamber and at least one of the operating-force relatingamount and the rate of increase in the operating-force relating amount.For example, the degree of opening of the throttle valve may bedecreased when the negative pressure in the first pressure chamber issmaller than a preset negative pressure and when the operating-forcerelating amount is greater than a preset amount, or when theoperating-force relating amount is greater than a preset amount and therate of increase in the operating-force relating amount is greater thana preset rate. In any case, the degree of opening of the throttle valveis decreased only when the negative pressure in the first pressurechamber is required to be increased. Thus, a frequency of operations fordecreasing the degree of opening of the throttle valve can be decreased.

Additionally, there is provided according another aspect of the presentinvention a vacuum booster apparatus adapted to be used for a brakeapparatus provided in a vehicle having an internal combustion engine,the vacuum booster apparatus comprising:

a vacuum booster having a first pressure chamber and a second pressurechamber, the first pressure chamber being connected to an intake pipe ofthe engine on a downstream side of a throttle valve, the second pressurechamber being selectively connectable to one of the first pressurechamber and atmosphere, the vacuum booster boosting an operating forceapplied to a brake operating member based on a pressure differencebetween the first pressure chamber and the second pressure chamber; and

a throttle-opening-degree control device controlling a negative pressurein the first pressure chamber by controlling a degree of opening of thethrottle valve,

wherein the throttle-opening-degree control device includeslarge-change-time opening degree decreasing means for decreasing thedegree of opening of the throttle valve as a rate of increase in anoperating-force relating amount related to the operating force appliedto the brake operating member is increased.

The negative pressure in the first pressure chamber needed to beincreased more when the rate of increase in the operating-force relatingamount is large as compared to when the rate of increase is small.

In the conventional vacuum booster apparatus, the degree of opening ofthe throttle valve is decreased when an operation of the brake operatingmember continues longer than a preset time and when the negativepressure in the first pressure chamber is smaller than a preset negativepressure. However, when a rapid braking operation is performed, that is,when the rate of increase in the operating force relating amount islarge, the negative pressure in the first pressure chamber must beincreased even if the operation of the brake operating member continuesa short time. Yet, in such a case, the degree of opening of the throttlevalve is not decreased on the other hand, when the rate of increase inthe operating-force relating amount is small, that is, for example, whenthe operating force applied to the brake operating member is released(the rate of increase is a negative value), the degree of opening of thethrottle valve is decreased although the negative pressure in the firstpressure chamber could be small. In order to eliminate such aninconvenience, in the vacuum booster apparatus according to thisinvention, the degree of opening of the throttle valve when the rate ofincrease in the operating-force relating amount is large is decreasedmore than when the rate of increase in the operating force relatingamount is small. Thus, the degree of opening of the throttle valve isdecreased only when it is necessary.

In the above-mentioned invention, the throttle-opening-degree controldevice may further include increasing-rate-related limited-timeopening-degree decreasing means for decreasing the degree of opening ofthe throttle valve only for a limited time when the negative pressure inthe first pressure chamber is smaller than a predetermined necessarynegative pressure determined according to the rate of increase in theoperating-force relating amount.

Additionally, there is provided according to another aspect of thepresent invention a vacuum booster apparatus adapted to be used for abrake apparatus provided in a vehicle having an internal combustionengine, the vacuum booster apparatus comprising:

a vacuum booster having a first pressure chamber and a second pressurechamber, the first pressure chamber being connected to an intake pipe ofthe engine on a downstream side of a throttle valve, the second pressurechamber being selectively connectable to one of the first pressurechamber and atmosphere, the vacuum booster boosting an operating forceapplied to a brake operating member based on a pressure differencebetween the first pressure chamber and the second pressure chamber; and

a throttle-opening-degree control device controlling a negative pressurein the first pressure chamber by controlling a degree of opening of thethrottle valve,

wherein the throttle-opening-degree control device includesoperating-time opening-degree-decrease suppressing means for suppressinga decrease in the degree of opening of the throttle valve so that thedecrease in the degree of opening of the throttle valve when the brakeoperating member is being operated is smaller than when the brakeoperating member is not being operated.

When a braking operation is being performed, the second pressure chamberis alternately connected to either the first pressure chamber oratmosphere. Accordingly, the negative pressure in the first pressurechamber is decreased as the braking operation continues. If the negativepressure in the first pressure chamber is sufficiently increased beforethe braking operation is performed, that is, when a braking operation isnot being performed, the negative pressure in the first pressure chambercan be maintained at a sufficient level for a long time. That is, thenegative pressure in the first pressure chamber can be prevented frombeing smaller than the predetermined necessary negative pressure if thenegative pressure in the first pressure chamber is increased when abraking operation is not being performed. Accordingly, a frequency ofoperations for decreasing the degree of opening of the throttle valvecan be deceased when a braking operation is being performed.

As discussed above, if the degree of opening of the throttle valve isdecreased when a braking operation is being performed, that is, theultra lean burn is performed, a rate of fuel consumption is increased.However, according to the vacuum booster apparatus of this invention,since a frequency of operations for decreasing the degree of opening ofthe throttle valve is decreased, an increase in the rate of fuelconsumption can be suppressed.

Additionally, when the negative pressure in the first pressure chamberis increased when a braking operation is being performed, a brakingforce is increased if the operating force applied to the brake operatingmember by a driver is constant. In order to maintain a constant brakingforce, the driver must control the operating force. This deteriorates abrake feel given to the driver. In order to eliminate such aninconvenience, in the vacuum booster apparatus according to thisinvention, a frequency of operations for decreasing the degree ofopening of the throttle valve is decreased so that the operation fordecreasing the degree of opening of the throttle valve is rarelyperformed when a braking operation is being performed. Thus, there is anadvantage in that deterioration of a braking feel is prevented.

In the vacuum booster apparatus according to the above-mentionedinvention, the throttle-opening-degree control device may includeopening-degree decreasing means for decreasing the degree of opening ofthe throttle valve when the negative pressure in the first pressurechamber is smaller than a preset negative pressure, and theoperating-time opening-degree-decrease suppressing means may includeoperating-time preset-negative-pressure decreasing means for decreasingthe preset negative pressure so that the preset negative pressure whenthe brake operating member is being operated is smaller than that whenthe brake operating member is not being operated.

When a braking operation is not being performed, the degree of openingof the throttle valve is decreased when the negative pressure in thefirst pressure chamber is smaller than a non-operating-time presetnegative pressure. When a braking operation is being performed, thedegree of opening of the throttle valve is decreased when the negativepressure in the first pressure chamber is smaller than an operating-timepreset negative pressure, which is smaller than the non-operating-timepreset negative pressure. Since the operating-time preset negativepressure is smaller than the non-operating-time preset negativepressure, a start condition to start a decreasing operation for thedegree of opening of the throttle valve when a braking operation isbeing performed tends to be unsatisfied as compared to that when abraking operation is not being performed. That is, a frequency ofoperations for decreasing the degree of opening of the throttle valve isdecreased since the non-operating-time preset negative pressure is setto be a relatively large value.

Each of the operating-time preset negative pressure and thenon-operating-time preset negative pressure may be either a fixed valueor a variable value determined according to an operational state of thebrake operating member.

For example, the operating-time preset negative pressure may be set to avalue at which the vacuum booster does not reach its boost limit whenthe brake operating member is operated by a normal operating force, andthe non-operating-time preset negative pressure may be set to a value atwhich the vacuum booster does not reach its boost limit even when thenegative pressure in the first pressure chamber becomes small due to thebrake operating member being operated by a normal operating force. Insuch a case, the non-operating-time preset negative pressure is set tobe greater than the operating-time preset negative pressure by an amountof decrease in the negative pressure when a normal braking operation isperformed.

Additionally, the operating-time preset negative pressure may be set tothe above-mentioned predetermined necessary negative pressure. Thenon-operating-time preset negative pressure may be a fixed value or avariable value. For example, the non-operating-time preset negativepressure may be set to a value greater than a maximum operating-timepreset negative value during a previous braking operation by theabove-mentioned amount of decrease in the negative pressure when anormal braking operation is performed. Additionally, thenon-operating-time preset negative pressure may be set to a valuecorresponding to a running state of the vehicle. That is, thenon-operating-time preset negative pressure when a vehicle speed islarge may be set to be greater than that when the vehicle speed is low,or the non-operating-time preset negative pressure when a vehicle isrunning up a slope may be set to be greater than that when the vehicleis running down a slope.

Additionally, in the vacuum booster apparatus according to theabove-mentioned invention, the operating-time preset-negative-pressuredecreasing means may include operating-time preset-negative-pressuredetermining means for determining the preset negative pressure used whenthe brake operating member is being operated in accordance with at leastone of an operating-force relating amount related to the operating forceapplied to the brake operating member and a rate of increase in theoperating-force relating amount.

When a braking operation is being performed, the degree of opening ofthe throttle valve is decreased only when the negative pressure in thefirst pressure chamber is smaller than the operating-time presetnegative pressure (hereinafter referred to as a variable preset negativepressure) which is determined based on at least one of theoperating-force relating amount and the rate of increase in theoperating-force relating amount. When the variable preset negativepressure is smaller than the operating-time preset necessary negativepressure (hereinafter referred to as a fixed preset negative pressure)which is previously determined as mentioned above, a frequency ofoperations for decreasing the degree of opening of the throttle valvecan be further decreased.

Additionally, the operating-time preset negative pressure may beselectively set to one of the variable preset negative pressure and thefixed preset negative pressure. For example, if the degree of opening ofthe throttle valve is decreased when the negative pressure in the firstpressure chamber becomes smaller than the one of the variable presetnegative pressure and the fixed preset negative pressure which isgreater than the other, the vacuum booster can be prevented fromreaching its boost limit when the brake operating member is beingoperated with a large operating force. In such a case, the fixed presetnegative pressure can be determined to be a minimum negative pressure inthe first pressure chamber.

Additionally, there is provided according to another aspect of thepresent invention a brake apparatus adapted to be provided in a vehiclehaving an internal combustion engine, the brake apparatus comprising:

a vacuum booster having a first pressure chamber and a second pressurechamber, the first pressure chamber being connected to an intake pipe ofthe engine on a downstream side of a throttle valve, the second pressurechamber being selectively connectable to one of the first pressurechamber and atmosphere, the vacuum booster boosting an operating forceapplied to a brake operating member based on a pressure differencebetween the first pressure chamber and the second pressure chamber;

a braking-force assisting apparatus increasing a braking force to avalue greater than a value corresponding to an output of the vacuumbooster; and

a throttle-opening-degree control device controlling a degree of openingof the throttle valve, the throttle-opening-degree control deviceincluding abnormal-time throttle-opening-degree decreasing means fordecreasing the degree of opening of the throttle valve, when anabnormality occurs in the braking force assisting apparatus, so as toincrease a negative pressure in the first pressure chamber.

In the above-mentioned invention, the braking-force assisting apparatusmay increase a braking force after the vacuum booster reaches its boostlimit or before the vacuum booster reaches its boost limit. As for thebraking-force assisting device which increases a braking force beforethe vacuum booster reaches its boost limit, there is a type whichincreases a braking force when an emergency braking operation isperformed and a type which increases a braking force a predeterminedperiod before the vacuum booster reaches its boost limit. If the vacuumbooster is a type in which a boost ratio changes before the vacuumbooster reaches its boost limit, a braking force is increased at a timewhen the boost ratio is changed. In any case, in a brake apparatusprovided with a braking-force assisting apparatus, a necessity fordecreasing the degree of opening of the throttle valve is high when anabnormality occurs in the braking-force assisting apparatus, and is lowwhen the braking-force assisting apparatus is normal.

The braking-force assisting apparatus may comprise a reservoir, a pumpwhich pressurizes the brake fluid in the reservoir and provides thepressurized brake fluid to a brake cylinder, a motor for driving thepump, a pressure control device which can control the fluid pressure inthe brake cylinder by the brake fluid discharged by the pump andbraking-force controlling means for controlling the fluid pressure inthe brake cylinder by controlling the motor and the pressure controldevice. When an abnormality occurs in the motor or the pressure controldevice, or when the an abnormality occurs in an electric system such asin the braking-force controlling means, it is determined that anabnormality occurs in the braking-force assisting apparatus. Anabnormality occurring in the braking-force assisting apparatus can bedetected by an abnormality detecting device.

A description will now be given of a case in which a braking force isincreased after the vacuum booster reaches substantially an atmosphericpressure.

When the braking-force assisting apparatus is normal, a braking forcecan be increased after the vacuum booster reaches its boost limit.However, when an abnormality occurs in the braking force assistingapparatus, a braking force cannot be increased. In such a case, theboost limit of the vacuum booster can be increased by increasing thenegative pressure in the first pressure chamber. If the brake operatingmember is operated with a large operating force, the vacuum booster isprevented from reaching its boost limit and the operating force isboosted by the vacuum booster which results in an increase in thebraking force. On the other hand, when the braking-force assistingapparatus is normal, the braking force can be increased after the vacuumbooster reaches its limit without increasing the boost limit of thevacuum booster. Thereby, the negative pressure in the first pressurechamber is not required to be increased.

When the braking-force assisting apparatus is abnormal, the degree ofopening of the throttle valve may be decreased when the negativepressure in the first pressure chamber is smaller than the presetnegative pressure, or may be decreased either when the negative pressurein the first pressure chamber is smaller or greater than the presetnegative pressure. The above-mentioned preset negative pressure may be apredetermined fixed value or a variable value which is determined by theoperating-force relating amount related to the operating force appliedto the brake operating member.

The braking-force assisting apparatus may be any apparatus whichincreases a braking force after the vacuum booster reaches its boostlimit. That is, the braking-force assisting apparatus may be anapparatus which increases a braking force to a value corresponding to anoperating force applied to the brake operating member, or may be anapparatus which increases a braking force independent of an operatingforce applied to the brake operating force. Additionally, the fact thatthe vacuum booster has reached its boost limit can be detected by aboost-limit detecting device which detects the pressure in the secondpressure chamber having reached an atmospheric pressure or detects themaster cylinder pressure having reached a boost limit which can beobtained according to the negative pressure in the first pressurechamber.

It should be noted that even in the brake apparatus according to thepresent invention, a decrease of the degree of opening of the throttlevalve cannot be completely omitted when the braking-force assistingapparatus is normal. When the braking force assisting apparatus isabnormal, the degree of opening of the throttle valve is decreased dueto an abnormality occurring in the braking-force assisting apparatus.The degree of opening of the throttle valve may be decreased, when thebraking-force assisting apparatus is normal, due to other causes such asa case in which the negative pressure in the first pressure chamber issmaller than the predetermined necessary negative pressure which isdetermined according to the operating-force relating amount related tothe operating force applied to the brake operating force. In such acase, when the braking-force assisting apparatus is abnormal, the degreeof opening of the throttle valve is decreased by the abnormal-timethrottle opening degree decreasing means, and when the braking-forceassisting apparatus is normal, the degree of opening of the throttlevalve is decreased by the limited-time opening-degree decreasing means.In a case in which a braking force is increased after the vacuum boosterreaches its boost limit, if the boost limit is increased when thebraking force assisting apparatus is normal, a frequency of the vacuumbooster reaching its boost limit can be decreased which results in adecrease in a frequency of operation of the braking-force assistingapparatus.

In the brake apparatus according to the present invention, thebraking-force assisting apparatus may increase the braking force after apressure in the second pressure chamber reaches substantially anatmospheric pressure, and the abnormal-time throttle-opening-degreedecreasing means may determine the degree of opening of the throttlevalve in accordance with a target boost limit.

According to this invention, the degree of opening of the throttle valvewhich degree is decreased when the braking-force assisting apparatus isabnormal, is determined by the abnormal-time throttle-opening-degreedetermining means. During an abnormal time in which the throttle valveis maintained at a degree of opening, the negative pressure in the firstpressure chamber when the abnormal-time throttle opening degree is smallcan be greater than that when the abnormal-time throttle opening degreeis large. That is, the abnormal-time throttle-opening degree when thetarget boost limit (corresponds to the negative pressure in the firstpressure chamber) is large is determined to be smaller than that whenthe target boost limit is small.

Additionally, together with or instead of the abnormal-time throttleopening degree, a decreased-state maintaining time which is a period formaintaining a state in which the degree of opening of the throttle valveis small may be determined. A level of the negative pressure in thefirst pressure chamber is not increased, immediately after the degree ofopening of the throttle valve is decreased, to a level corresponding tothe degree of opening of the throttle valve. Accordingly, the negativepressure in the first pressure chamber and the boost limit when thedecreased-state maintaining time is long is greater than that when thedecreased-state maintaining time is short. In such a case, theabnormal-time throttle-opening-degree decreasing means includesopening-degree decreased-state-maintaining determining means.

In the brake apparatus according to the present invention, thebraking-force assisting apparatus may increase the braking force after apressure in the second pressure chamber reaches substantially anatmospheric pressure, and the abnormal-time throttle-opening-degreedecreasing means decreases the degree of opening of the throttle valvewhen the negative pressure in the first pressure chamber is smaller thana predetermined necessary negative pressure which is determined inaccordance with at least one of an operating-force relating amountrelated to the operating force applied to the brake operating member anda rate of increase in the operating-force relating amount.

In the brake apparatus according to this invention, the degree ofopening of the throttle valve is decreased when the braking-forceassisting apparatus is abnormal and when the negative pressure of thefirst pressure chamber is smaller than the predetermined necessarynegative pressure. That is, if the braking-force assisting apparatus isabnormal, the negative pressure in the first pressure chamber is notrequired to be increased when the vacuum booster does not reach itsboost limit or when a possibility for the vacuum booster reaching itsboost limit is low. Thus, in the brake apparatus according to thepresent invention, a frequency of operations for decreasing the degreeof opening of the throttle valve can be decreased.

Additionally, in the brake apparatus according to the present invention,the abnormal-time throttle-opening-degree decreasing means may decreasethe degree of opening of the throttle valve when the brake operatingmember is not being operated and when the negative pressure in the firstpressure chamber is smaller than a non-operating-time preset negativepressure, and the abnormal-time throttle opening degree decreasing meansmay also decrease the degree of opening of the throttle valve when thebrake operating member is being operated and when the negative pressurein the first pressure chamber is smaller than an operating-time presetnegative pressure which is smaller than the non-operating-time presetnegative pressure.

As mentioned above, the degree of opening of the throttle valve does nothave to be decreased immediately after an abnormality occurs in thebraking-force assisting apparatus, and may be decreased when thenegative pressure in the first pressure chamber becomes smaller than thenon-operating time preset negative pressure or the operating-time presetnegative pressure.

Additionally, in the brake apparatus according to the present invention,the throttle-opening-degree control device may include normal-timeconditioned throttle-opening-degree decreasing means for decreasing thedegree of opening of the throttle valve when the braking-force assistingapparatus is normal and when the negative pressure in the first pressurechamber satisfies a predetermined condition.

According to this invention, the degree of opening of the throttle valveis decreased by the abnormal-time throttle-opening-degree decreasingmeans when the braking force assisting apparatus is abnormal, and thedegree of opening of the throttle valve is decreased by the normal-timeconditioned throttle-opening-degree decreasing means when thebraking-force assisting apparatus is normal. The normal-time conditionedthrottle-opening-degree decreasing means may include at least one of thelimited-time opening-degree decreasing means, the large-change-timeopening-degree decreasing means and the operating-timeopening-degree-decrease suppressing means.

In a case in which the braking-force assisting apparatus increases abraking force after the pressure in the second pressure chamber reachessubstantially an atmospheric pressure, a frequency of operations of thebraking-force assisting apparatus can be decreased if the boost limit ofthe vacuum booster is increased by decreasing the degree of opening ofthe throttle valve when the braking-force assisting apparatus is normal.Thus, when the braking force assisting apparatus includes a pump, anoperating noise of the pump can be reduced.

Additionally, there is provided according to another aspect of thepresent invention a brake apparatus adapted to be provided in a vehiclehaving an internal combustion engine, the brake apparatus comprising:

a vacuum booster having a first pressure chamber and a second pressurechamber, the first pressure chamber being connected to an intake pipe ofthe engine on a downstream side of a throttle valve, the second pressurechamber being selectively connectable to one of the first pressurechamber and atmosphere, the vacuum booster boosting an operating forceapplied to a brake operating member based on a pressure differencebetween the first pressure chamber and the second pressure chamber so asto output the boosted operating force to a master cylinder;

a brake cylinder operated by a fluid pressure generated by the mastercylinder;

a pressure-increasing device increasing a fluid pressure provided to thebrake cylinder to be greater than the fluid pressure generated by themaster cylinder; and

a throttle-opening-degree control device controlling a degree of openingof the throttle valve, the throttle-opening-degree control deviceincluding abnormal-time throttle-opening-degree decreasing means fordecreasing the degree of opening of the throttle valve, when anabnormality occurs in the pressure-increasing device, so as to increasea negative pressure in the first pressure chamber.

The above-mentioned pressure-increasing device can be a braking-forceassisting apparatus which increases a braking force after the vacuumbooster reaches its boost limit or an emergency-time braking-forceassisting apparatus which increases a braking force at an emergencytime. It is preferable that in a case in which the emergency-timebraking-force is used, when the emergency-time braking force assistingapparatus is abnormal, the negative pressure in the first pressurechamber be set to be greater than a non-operating-time preset negativepressure used when a braking operation is not being performed. This isbecause a braking force can be increased when a large operating force isapplied by a driver at an emergency time.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram of a brake apparatus including a vacuumbooster apparatus according to a first embodiment of the presentinvention;

FIGS. 2A and 2B are illustrations for explaining a structure and anoperation of a pressure control valve shown in FIG. 1;

FIG. 3 is a structural diagram of an engine provided with a throttlevalve which controls a negative pressure used by a vacuum booster shownin FIG. 1;

FIG. 4 is a graph showing a relationship between a negative pressure anda boost limit in the vacuum booster shown in FIG. 1;

FIG. 5 is a graph showing a relationship between a pressure in a brakecylinder and a force applied to a brake pedal;

FIG. 6 is a graph showing a relationship between a pressure differenceand a master cylinder pressure;

FIG. 7 is a graph for explaining an output of the vacuum booster when anegative pressure is increased;

FIG. 8 is a flowchart of a throttle-opening-degree control operation;

FIG. 9 is a flowchart of a process in step S14 of FIG. 8;

FIG. 10 is a flowchart of a braking-force control operation;

FIG. 11 is a diagram for explaining an operation of the vacuum booster;

FIG. 12 is a flowchart of an operation according to athrottle-opening-degree control program stored in a ROM of a brakecontrol apparatus included in the brake apparatus shown in FIG. 1;

FIG. 13 is a flowchart of an operation according to athrottle-opening-degree control program stored in a ROM of a brakecontrol apparatus included in a brake apparatus according to a secondembodiment of the present invention; and

FIG. 14 is a graph showing a relationship between a throttle openingdegree and a target pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to FIG. 1, of a firstembodiment of the present invention. FIG. 1 is a structural diagram of abrake apparatus including a vacuum booster apparatus according to thefirst embodiment of the present invention.

The brake apparatus shown in FIG. 1 comprises a vacuum booster apparatus6 and a braking-force assisting apparatus 8. A vacuum booster 12included in the vacuum booster apparatus 8 has a limit of boost.Considering such a limit of boost, the braking-force assisting apparatus8 controls a braking force so that a deceleration of a vehicle increaseswith an ideal slope relative to a brake operating force. Thebraking-force assisting apparatus 8 is operated by a pump 40. In thebrake apparatus, an antilock control can be performed by using the pump40 so as to prevent a tendency of lock of each wheel from beingexcessive when a braking operation is being performed.

In FIG. 1, a brake pedal 10 corresponds to a brake operating member. Thebrake pedal 10 is connected to a master cylinder 14 via the vacuumbooster 12.

In the vacuum booster 12 (hereinafter referred to as booster 12), apower piston is movably provided within a housing having an inner spaceso as to form an airtight seal with an inner wall of the housing. Theinner space of the housing of the vacuum booster 12 is divided by thepower piston into a negative pressure chamber 12 a corresponding to afirst pressure chamber and a pressure changing chamber 12 bcorresponding to a second pressure chamber. The negative pressurechamber 12 a is located on the master cylinder 14 side. The pressurechanging chamber 12 b is located on the brake pedal 10 side. Thenegative pressure chamber 12 a is connected to an intake pipe 18 of anengine at a position downstream of a throttle valve 20 as shown in FIG.3.

The master cylinder 14 is a tandem type which includes two pistonsserially arranged and slidably fit within a housing so that the twopistons are moved by an output of the booster 12. The master cylinder 14provides a the same fluid pressure to each of chambers formed in frontof the two pistons. A brake cylinder 26 operating a front left wheel FLand a brake cylinder 26 operating a rear right wheel RR are connected toone of the chambers, and a brake cylinder operating a front right wheelFR and a brake cylinder operating a rear left wheel RL are connected tothe other of the chambers. The brake apparatus can be of a disc type ora drum type which restricts rotation of a wheel by pressing a frictionmember against a friction surface of a rotating member rotating with thewheel, a pressing force of the friction member being generated by afluid pressure.

A fluid passage 24 extends from one of the chambers of the mastercylinder 14. The fluid passage 24 is branched and the brake cylinders 26of the front left and rear right wheels FL and RR are connected torespective ends of the branched fluid passage 24.

A pressure control valve 30 is provided on a master cylinder 14 side ofa branched point of the fluid passage 24. The pressure control valve 30controls a fluid pressure on the brake cylinder 26 side of the fluidpassage 24 relative to a fluid pressure on the master cylinder 14 side.Specifically, if a pressure difference between the brake cylinderpressure and the master cylinder pressure is smaller than a targetpressure difference in a state in which a brake fluid is discharged fromthe pump 40, the pressure control valve 30 prevents a flow of the brakefluid from the pump 40 to the master cylinder 14. On the other hand, ifthe pressure difference tends to be greater than the target pressuredifference, the pressure control valve 30 permits the flow of brakefluid from the pump 40 to the master cylinder 14. Accordingly, the brakecylinder pressure is controlled by the pressure control valve 30 so thatthe brake cylinder pressure is greater than the master cylinder pressureand the pressure difference therebetween is equal to the target pressuredifference.

The pressure control valve 30 of the present embodiment is a type thatelectromagnetically controls the pressure difference between pressuresin the brake cylinders 26 and the master cylinder 14. Specifically, asshown in FIGS. 2A and 2B, the pressure control valve 30 comprises ahousing (not shown in the figure), a valve body 70, a valve seat 72 onwhich the valve body 70 sits and a solenoid 74 which generates amagnetic force so as to move the valve body 70 relative to the valveseat 72. The valve body 70 controls a state of the brake fluid flow inthe fluid passage 24 between the master cylinder 14 and the brakecylinder 26.

In a non-operated state (OFF state) in which the solenoid 74 is notactivated, the valve body 70 is separated from the valve seat 72 by aspring force of a spring 76. Thereby, flow of the brake fluid in thefluid passage 24 between the master cylinder 14 and the brake cylinders26 is permitted in both directions. As a result, when a brakingoperation is performed, the brake cylinder pressure is changed inresponse to a change in the master cylinder pressure. During the brakingoperation, a force is exerted on the valve body 70 in a direction inwhich the valve body 70 is separated from the valve seat 72.Accordingly, as long as the solenoid 74 is not activated, the valve body70 does not sit on the valve seat 72 even if the master cylinderpressure, that is, the brake cylinder pressure, is increased. That is,the pressure control valve 30 is a normally open valve.

On the other hand, in an operated state (ON state) in which the solenoid74 is activated, an armature 78 is attracted by a magnetic force of thesolenoid 74, and the valve body 70 (movable member) which moves togetherwith the armature 78 is seated on the valve seat 72 (stationary member).At this time, as shown in FIG. 2B, the attracting force F1 and a forceof a sum of forces F2 and F3 are exerted on the valve body 70 inopposite directions. The attracting force F1 is generated by themagnetic force generated by the solenoid 74. The force F2 is generatedbased on a pressure difference between the brake cylinder pressure andthe master cylinder pressure. The spring force F3 is generated by thespring 76. A magnitude of the force can be represented by a product ofthe pressure difference between the brake cylinder pressure and themaster cylinder pressure and an effective pressure receiving area of thevalve body 70 which area receives the brake cylinder pressure.

In the operated state (ON state) in which the solenoid 74 is activated,and the discharge pressure of the pump 40, that is, the brake cylinderpressure, is sufficiently low such that a relationship F2≦F1-F3 isestablished, the valve body 70 is maintained to be seated on the valveseat 72. That is, the valve body 70 is maintained to be seated on thevalve seat 72 when the pressure difference force F2 does not exceed thesum of the attracting force F1 and the spring force F3. In such a state,the brake fluid is prevented from flowing from the pump 40 to the mastercylinder 14. Accordingly, the discharge pressure of the pump 40 isincreased, and a fluid pressure higher than the master cylinder pressureis generated in the brake cylinders 26.

If the discharge pressure of the pump 40, that is, the brake cylinderpressure, is further increased such that a relationship F2>F1-F3 isestablished, the valve body 70 is separated from the valve seat 72. Thatis, when the sum of the pressure difference force F2 and the springforce F3 exceeds the attracting force F1, the valve body 70 is separatedfrom the valve seat 72, and the brake fluid is provided from the pump 40to the master cylinder 14. As a result, the discharge pressure of thepump 40, that is, the brake cylinder pressure, is prevented from beingfurther increased. Accordingly, if the spring force F3 of the spring 76is not considered, a brake cylinder pressure greater than the mastercylinder pressure by the attracting force F1 of the solenoid isgenerated.

Additionally, the pressure control valve 30 is designed so that amagnitude of the attracting force F1 of the solenoid 74 changes linearlyin response to a magnitude of an activating current I of the solenoid74.

As shown in FIG. 1, a bypass passage 82 is provided to the pressurecontrol valve 30. A check valve 84 is provided in the middle of thebypass passage 82. The check valve 84 is provided so as to keep a fluidflow from the master cylinder 14 to the brake cylinders 26 even if thepressure control valve 30 is closed due to a force exerted on themovable member in the pressure control valve 30 when the brake pedal 10is being pressed. Additionally, a relief valve 86 is provided parallelto the check valve 84 so as to prevent a pressure built by the dischargepressure of the pump 40 from being excessive.

Pressure increasing valves 90, each of which is a normally open solenoidvalve, are provided to the branched portion of the fluid passage 24 soas to achieve a pressure increasing state in which the brake fluid isprevented from flowing from the master cylinder 14 to the brakecylinders 26 when the pressure increasing valves 90 are closed. A bypasspassage 92 is connected to each of the pressure increasing valves 90,and a check valve 94 is provided to each of the bypass passages 92. Areservoir passage 96 extends from a position between each of thepressure increasing valves 90 and the respective brake cylinder 26, andthe reservoir passage reaches a reservoir 98. A pressure decreasingvalve 100, which is a normally closed solenoid valve, is provided in themiddle of each reservoir passage 96 so as to achieve a pressuredecreasing state in which a flow of the brake fluid from the brakecylinders 26 to the reservoir 98 is permitted when the pressuredecreasing valve 100 is open.

The reservoir 98 comprises a housing and a reservoir piston 104 slidablyfit in the housing so a to form a substantially airtight seal with aninner wall of the housing. A reservoir chamber 106 is formed by thehousing and the reservoir piston 104 so as to store the brake fluid inthe reservoir chamber 106 under a pressure generated by a spring 108.

The reservoir 98 is connected to an inlet of the pump 40 via an inletpassage 110. An output of the pump 40 is connected to a portion of thefluid passage 24 between the pressure control valve 30 and the pressureincreasing valves 90 via an outlet passage 114. An inlet valve 116 whichis a check valve is provided to the inlet passage 110. An outlet valve118 which is a check valve is provided to the outlet passage 114.Additionally, an orifice 120 and a fixed damper 122 are provided to theoutlet passage 114 so as to reduce pulsation in the brake fluidpressure.

In the present embodiment, a supplementary passage 130 is provided toconnect the master cylinder 14 and to the reservoir 98. A flow controlvalve 140 is provided to the supplementary passage 130. The flow controlvalve 140 is a normally closed solenoid valve which is opened when thebrake fluid should be provided from the master cylinder 14 to thereservoir 98 and when the pump 40 is being operated so that the brakefluid flows from the master cylinder 14 to the reservoir 98. When it isnot required to provide the brake fluid from the master cylinder 14 tothe reservoir 98, the flow control valve 140 is closed so as to preventthe brake fluid from flowing from the master cylinder 14 to thereservoir 98, which flow permits an increase in the fluid pressure bythe master cylinder 14. It is determined whether or not the brake fluidis required to be introduced from the master cylinder 14 based on adetermination as to whether or not the brake fluid is present in thereservoir 98, which brake fluid is to be pumped up by the pump 40. Whenan antilock control is being performed, an accumulation of periodsduring which the pressure increasing valve 90 achieves the pressureincreasing state is calculated. Additionally, an accumulation of periodsduring which the pressure decreasing valve 100 achieves the pressuredecreasing state is calculated. An amount of brake fluid remaining inthe reservoir 98 is estimated based on the pressure increasing time andthe pressure decreasing time. As mentioned above, the flow control valve140 is opened when the pump 40 is being operated and there is no brakefluid to be pumped up by the pump 40. Thereby, the brake fluid isprevented from being provided from the master cylinder 14 to thereservoir before the master cylinder pressure is sufficiently increased.Additionally, the reservoir 98 is prevented from storing an excessiveamount of brake fluid due to the brake fluid in the master cylinder 14being directly suctioned during an antilock control.

The supplementary passage 130 does not directly connect the mastercylinder 14 and the reservoir 98. That is, the supplementary passage 130is connected to a middle portion of the inlet passage 110. A check valve142 is provided between the middle portion and the reservoir 98 so as toprevent the brake fluid from flowing from the supplementary passage 130to the reservoir 98. During a braking operation, before the brake fluidin the master cylinder 14 is provided to the reservoir 98 through thecheck valve 142 by the flow control valve 30 being opened, the brakefluid in the reservoir 98 can be pumped up by the pump 40. Accordingly,a quick response of the pump 40 can be achieved than pumping the brakefluid after a pressure in the reservoir 98 is decreased. Additionally, aload applied to the pump 40 is reduced which facilitates a reduction ina capacity of the pump 40.

The brake apparatus is controlled by a brake control apparatus 144. Thebrake control apparatus 144 comprises a microcomputer. An input of themicro computer is connected to a brake switch 150. Other inputs of themicrocomputer are connected to a master-cylinder-pressure sensor 152,wheel speed sensors 156 and an abnormality detecting device 158. Thebrake switch 150 detects whether the brake pedal 10 is in an operatedstate. The master-cylinder-pressure sensor 152 detects the mastercylinder pressure. The wheel speed sensor 156 detects a rotational speedof each wheel. The abnormality detecting device 158 detects whether thebraking-force assisting apparatus 8 is normal or abnormal. An output ofthe microcomputer is connected to the solenoid 74 of the pressurecontrol valve 30 and a solenoid of each of the solenoid valves 90, 100and 140 via respective drive circuits. A ROM provided in the brakecontrol apparatus 144 stores various sets of information including tableinformation shown in FIG. 4 which table information indicates arelationship between a negative pressure in the negative pressurechamber 12 a and a limit of boost, programs for controlling a degree ofopening of the throttle valve 20 represented by flowcharts shown inFIGS. 8 and 9 and programs for controlling a braking force representedby a flowchart shown in FIG. 10.

The above-mentioned throttle valve 20 is provided in the engine shown inFIG. 3. The engine shown in FIG. 3 is a direct injection gasoline enginewhich is provided with a high-pressure swirl injector 164 so as todirectly inject gasoline into a cylinder 166. A piston 168 is movedwithin the cylinder 166 due to combustion of gasoline, and, thereby, acrank 170 is driven.

The above-mentioned intake pipe 18 is connected to the cylinder 166 sothat air is introduced into the cylinder 166 through the intake pipe 18.The throttle valve 20 and a swirl valve 174 are provided in the intakepipe 18. A vacuum hose 22 is connected to the intake pipe between thethrottle valve 20 and the swirl valve 174. The throttle valve 20 isoperated by a throttle motor 176 so that a degree of opening of thethrottle valve 20 is controlled by controlling an operation of thethrottle motor 176. An amount of intake air introduced into the cylinder166 is controlled by controlling a degree of opening of the throttlevalve 20. A negative pressure in the negative pressure chamber 12 a ofthe booster 12 is also controlled by controlling the degree of openingof the throttle valve 20. The degree of opening of the throttle valve 20is detected by a throttle position sensor 178. Similarly, the swirlvalve 174 is controlled by an operation of a swirl motor 180. The degreeof opening of the swirl valve 174 is detected by an SCV position sensor182. The degree of opening of the swirl valve 174 is controlled so thata swirl of air (air flow) in the cylinder 166 is optimum for a state ofcombustion.

An exhaust pipe 190 is also connected to the cylinder 166. A gasgenerated in the combustion chamber is exhausted to atmosphere via athree-state catalytic converter.

An intake valve 192 and an exhaust valve 194 are provided to the intakepipe 18 and an opening of the exhaust pipe 190 at the cylinder 166,respectively. The intake valve 192 and the exhaust valve 194 areoperated by rotation of a camshaft. In the present embodiment, a timingof the intake valve 192 is controllable.

An exhaust gas cleaning device is provided to a connecting passage whichconnects between the exhaust pipe 190 and the intake pipe 18 bybypassing the cylinder 166 so as to recirculate an exhaust, which iscontrolled in response to an operational state of the engine so that anamount of nitrogen oxide NOx is reduced. An amount of exhaust gas to berecirculated is controlled by an exhaust gas recirculation (EGR) valve196.

The high-pressure swirl injector 164 is connected to a fuel tank 208 viaa low-pressure pump 202, a high-pressure pump 204 and a high-pressurepipe 206. The gasoline stored in the fuel tank 208 is pumped by thelow-pressure pump 202 and is pressurized by the high-pressure pump 204so that a predetermined amount of pressurized gasoline is provided tothe high-pressure swirl injector 164. The high-pressure swirl injector164 is provided with a swirl nose from which gasoline is injected intothe cylinder 166 so that the injected gasoline becomes fine mist. A fuelpressure sensor 210 is mounted in the middle of the high-pressure pipe206 so as to detect a pressure of the gasoline to be provided to thehigh-pressure swirl injector 164. Additionally, gasoline pumped by thelow-pressure pump 202 is provided to a low-temperature injector 212provided to a middle portion of the intake pipe 18.

The high-pressure pump 204 is driven by a cam which operates the intakevalve 192 so that gasoline is provided in association with an operationof the cam. The high-pressure pump 204 includes a solenoid valve so thatan amount of gasoline delivered by the high-pressure pump 204 iscontrollable according to a state of activation of a solenoid of thesolenoid valve.

The engine shown in FIG. 3 is controlled by an engine control apparatus220. The engine control apparatus 220 comprises a microcomputer. Inputsof the microcomputer are connected to the throttle position sensor 178,the SCV position sensor 182 and the fuel pressure sensor 210.Additionally, an accelerator position sensor 230 which detects a degreeof opening of an acceleration pedal 228, a crank position sensor 232which detects a position of the crank 170 and a booster negativepressure sensor 234 which detects a negative pressure in the negativepressure chamber 12 a of the booster 12 are connected to inputs of themicrocomputer. An output of the microcomputer is connected to thethrottle motor 176 and the swirl motor 180 via respective drive circuits(not shown in the figure).

An engine revolution speed is detected based on a crank positiondetected by the crank position sensor 232. A drive torque desired by adriver is determined based on a degree of opening of the acceleratordetected by the accelerator position sensor 230. A ROM provided in theengine control apparatus 220 stores various programs including programsfor controlling the degree of opening of the throttle valve 20. In thepresent embodiment, an amount of fuel injected by the high-pressureswirl injector 164, the degree of opening of the throttle valve 20, adegree of opening of the swirl valve 174, the degree of opening of theEGR valve 196, an intake valve timing and an ignition timing arecontrolled based on the engine revolution speed and a degree of openingof the accelerator.

In the engine associated with the present embodiment, the ultra leanburn (stratified charge combustion) is performed in a state in which theengine speed is not very high (in a state in which an engine load is notvery large). On the other hand, a uniform combustion is performed in astate in which the engine speed is high (in a state in which the engineload is large). In the uniform combustion, a combustion is performed atan air-fuel ratio near a stoichiometric air-fuel ratio. However, in theultra lean burn, a combustion is performed at a high air-fuel ratio ofabout 25 to 50. In a state in which an engine load is not very large, anecessary torque can be output even when the ultra lean burn occurs.During a braking operation, since an engine load is not very large, theultra lean burn is performed in many cases. According to the ultra leanburn, a rate of fuel consumption is reduced which results in energysaving. During the ultra lean burn, a degree of opening of the throttlevalve 20 is set to a relatively large value because a large amount ofair is required to increase a air-fuel ratio in order to preventmisfire.

However, since the negative pressure chamber 12 a of the booster 12 isconnected to the downstream side of the throttle valve 20, a negativepressure in the negative pressure chamber 12 a is decreased when thedegree of opening of the throttle valve 20 is set to a relatively largevalue. If the brake pedal 10 is pressed in a state in which the negativepressure of the negative pressure chamber 12 a is small, the booster 12reaches its boost limit even if the operating force to the brake pedal10 is not very large. This cannot provide a sufficient boost action. Asshown in FIG. 4, when the negative pressure is decreased, the boostlimit is also decreased. In the present embodiment, to increase thenegative pressure of the negative pressure chamber 12 a, the degree ofopening of the throttle valve 20 is decreased. When the degree ofopening of the throttle valve 20 is decreased while the ultra lean burnis performed, the combustion is stepwisely or gradually changed from theultra lean burn to a uniform combustion so that the degree of opening ofthe throttle valve 20 is decreased in a state of uniform combustion.This is because if the degree of opening of the throttle, valve 20 isdecreased during the ultra lean burn, a state of combustion becomesunstable due to misfire or the like. Additionally, when the same outputtorque is output, a amount of injected fuel, the degree of opening ofthe throttle valve 20, the degree of opening of the swirl valve, and thedegree of opening of the EGR valve 196 are much different between theultra lean burn and the uniform combustion. Accordingly, if these valuesare rapidly changed, drivability is deteriorated. Additionally,combustion is changed to the uniform combustion when an abnormalityoccurs in the hydraulic brake apparatus. This includes a case in whichan abnormality occurs in the braking-force assisting apparatus 8.Additionally, the state of the engine is stepwisely or gradually changedfrom the state of uniform combustion to the state of ultra leancombustion.

The engine control apparatus 220 is connected to the brake controlapparatus 144 via a communication device so that exchange of informationis performed therebetween. Negative pressure information whichrepresents a negative pressure in the negative pressure chamber 12 adetected by the booster negative pressure sensor 234 is provided fromthe engine control apparatus 220 to the brake control apparatus 144.Opening-degree decreasing information which instructs the engine controlapparatus 220 to decrease of a degree of opening of the throttle valve20 and opening-degree control permitting information which instructs theengine control apparatus 220 to control the degree of opening areprovided from the brake control apparatus 144 to the engine controlapparatus 220. The opening-degree control permitting information isprovided when there is no need to decrease the degree of opening. Asmentioned above, when the degree of opening of the throttle valve 20 isdecreased during the ultra lean burn, a state of combustion must bechanged to the uniform combustion. Accordingly, there is a problem inthat a rate of fuel consumption is increased. In order to eliminate sucha problem, in the present embodiment, a frequency of decreasing thedegree of opening of the throttle valve 20 during the ultra lean burn isdecreased.

A description will now be given of an operation of the brake apparatusshown in FIG. 1.

When the brake pedal 10 is pressed, an operating force to the brakepedal 10 is boosted by the booster 12, and is transmitted to the mastercylinder 14. In the master cylinder 14, a fluid pressure correspondingto the operating force is generated in each of the pressurizingchambers, and the generated pressure is transmitted to the brakecylinders 26. If a tendency of locking of the left and right frontwheels is excessive, an antilock control is performed so as to maintainan appropriate brake slip state. That is, the pressure in the brakecylinders 26 is controlled by the solenoid valves 90 and 100 accordingto the pressure of the brake fluid discharged by the pump 40. It shouldbe noted that the pressure in the brake cylinders 26 may be controlledbased on a pressure of the brake fluid in the master cylinder 14.

Additionally, when the operating force applied to the brake pedal 10 isincreased and the booster reaches its boost limit, a pump motor 160 isoperated. Accordingly, the pressure in the brake cylinders 26 isincreased so as to be controlled to a value corresponding to theoperating force by a control of the solenoid 74 of the pressure controlvalve 30. A pressure higher than the master cylinder pressure P_(M) by apressure difference ΔP is generated in the brake cylinders 26. Thepressure-difference ΔP is an increased amount of the brake cylinderpressure P_(B) relative to the master cylinder pressure P_(M).Accordingly, as shown in FIG. 5, the braking force can be increased witha uniform increasing slope before and after the boost limit of thebooster 12 is reached. A relationship between the pressure difference ΔPand the master cylinder pressure P_(M) is as shown in a graph of FIG. 6A current supplied to the solenoid 74 of the pressure control valve 30is controlled so that the pressure difference shown in FIG. 6, isachieved.

The vacuum booster 12 cannot boost the operating force applied to thebrake-operating member after a pressure in the pressure changing chamber12 b increases from a negative pressure and reaches an atmosphericpressure. If the braking force is increased by a device other than thebooster 12 after the boost limit is reached, the braking force can beincreased with an appropriate slope (for example, with a substantiallyuniform slope). Thus, the braking force can be increased with a stableslope irrespective of whether or not the boost limit is reached. Thus,safeness and a feel of a braking operation can be improved.

In the present embodiment, the braking-force assisting apparatus 8 isconstituted by the reservoir 98, the pump 40, the pump motor 160, thepressure control valve 30, the flow control valve 140, the pump motor160 of the brake control apparatus 144 and units for controlling thepressure control valve 30 and the flow control valve 140.

Additionally, a determination as to whether or not the booster 12reaches its boost limit can be made based on the master cylinderpressure P_(M) and the negative pressure in the negative pressurechamber 12 a. Since the boost limit of the booster 12 is determined by apressure difference between the pressures in the negative pressurechamber 12 a and the pressure changing chamber 12 b when the pressure inthe pressure changing chamber 12 b reaches an atmospheric pressure, theboost limit P_(S) of the booster 12 can be estimated based on thenegative pressure in the negative pressure chamber 12 a as shown in FIG.4. If the master cylinder pressure P_(M) reaches the boost limit P_(S),it can be determined that the boost function of the booster 12 hasreached its limit. In the present embodiment, a boost-limit detectingunit is constituted by the master-cylinder-pressure sensor 152, thebooster negative pressure sensor 234 and parts for determining whetheror not the booster 12 of the brake control apparatus 144 reaches itsboost limit.

A control of the braking-force assisting apparatus 8 is performed inaccordance with a braking-force control program represented by aflowchart of FIG. 10.

In step S1, it is determined whether or not the booster 12 reaches itsboost limit. That is, it is determined whether or not the mastercylinder pressure P_(M) is equal to or greater than the boost limitP_(S). If it is determined that the boost limit has been reached, theroutine proceeds to step S2 so as to perform a pressure-increasingcontrol. On the other hand, if it is determined that the boost limit hasnot been reached, the routine proceeds to step S3 so as to perform thenon-pressure-increasing control.

In the pressure-increasing control, a current is supplied to thesolenoid 74 of the pressure control valve 30 so that the pressuredifference ΔP is achieved and the flow control valve 140 is opened ifnecessary. Additionally, the pump motor 160 is driven. On the otherhand, in the non-pressure-increasing control, the current supplied tothe solenoid 74 of the pressure control valve 30 is set to zero, and theflow control valve 140 is closed. Additionally, the pump motor 160 isstopped. It should be noted that the pump motor 160 may be drivenaccording to the antilock control program.

As mentioned above, when the braking force assisting apparatus 8 isnormal, the braking force after the booster 12 reaches the boost limitcan be increased with the same slope as the slope before the boosterlimit is reached. However, when an abnormality occurs in thebraking-force assisting apparatus 8, the braking force cannot beincreased with the same slope. In order to eliminate such a problem,when an abnormality is detected in the braking-force assisting apparatus8 and when the negative pressure in the negative pressure chamber 12 ais less than a predetermined negative pressure described later, theopening-degree decreasing information is output to the engine controlunit 220 so as to decrease the degree of opening of the throttle valve20. When the negative pressure in the negative pressure chamber 12 a isincreased, the boost limit of the booster 12 can be increased. Thus, ifthe brake pedal 10 is operated by a large operating force, the brakingforce can be sufficiently boosted by the booster 12. An abnormality ofthe braking-force assisting apparatus 8 can be detected by theabnormality detecting device 158. In the present embodiment, theabnormality detecting device 158 detects an abnormality occurring in anelectric system. It should be noted that the abnormality detectingdevice 158 may detect an abnormality occurring in a hydraulic systemsuch as fluid leakage in the inlet passage 110 or the outlet passage114.

The above-mentioned predetermined negative pressure is set to be anon-operating-time preset negative pressure β1 during a non-operatingtime, and is set to one of an operating-time preset negative pressure β2and a necessary negative pressure, whichever is greater, during anoperating time. The operating-time preset negative pressure β2 issmaller than the non-operating-time preset negative pressure β1; thatis, the operating-time preset negative pressure β2 is closer to anatmospheric pressure than the non-operating-time preset negativepressure β1. Accordingly, a frequency of operations for decreasing thedegree of opening of the throttle valve 20 when the brake pedal 10 isbeing operated is smaller than that when the brake pedal 10 is not beingoperated. When the brake pedal 10 is not being operated, the negativepressure chamber 12 a and the pressure changing chamber 12 b areconnected to each other so that the pressure in the pressure changingchamber 12 b is equal to the pressure in the negative pressure chamber12 a. When the brake pedal 10 is operated, the pressure changing chamber12 b is alternately connected to either the negative pressure chamber 12a or atmosphere. Thereby, the negative pressure in the negative pressurechamber 12 a is decreased. However, if the negative pressure issufficiently increased when the brake pedal is not operated, a conditioncan be established that the booster 12 does not reach its boost limiteven when the negative pressure is decreased while the brake pedal 10 isoperated. In the present embodiment, the non-operating-time presetnegative pressure β1 is set to a value (closer to an absolute vacuum),which is greater than a value at which the boost limit is not reachedwhen a brake operation is performed with a normal operating force bymore than an amount of decrease in the negative pressure when a normalbraking operation is performed. Additionally, the operating-time presetnegative pressure β2 is set to a value at which the boost limit is notreached by a normal braking operation.

During a braking operation, since an engine load is not very large, asmentioned above, an ultra lean combustion is performed normally.However, when the degree of opening of the throttle valve 20 isdecreased, the state of combustion must be changed to a uniformcombustion. This causes a problem in that a rate of fuel consumption isincreased. On the other hand, when a braking operation is not performed,there is a case in which the uniform combustion is performed. Thus, arate of fuel consumption may not be increased if the degree of openingof the throttle valve 20 is decreased. Accordingly, an increase in arate of fuel consumption can be suppressed by decreasing the degree ofopening of the throttle valve 20 when the brake pedal 10 is not beingoperated rather than when the brake pedal 10 is being operated.

When the negative pressure is increased when the braking operation isperformed, a braking force is increased if an operating force applied tothe brake pedal by a driver is maintained constant and a stroke of thebrake pedal is increased as shown in FIG. 7. On the contrary, in orderto maintain the braking force at a constant value, the driver mustdecrease the operating force applied to the brake pedal 10 whichdeteriorates a brake feel. However, in the present embodiment, afrequency of operations for decreasing the degree of opening of thethrottle valve 20 is decreased when the braking operation is performed,and, thereby, deterioration in the brake feel can be suppressed.

The above-mentioned necessary negative pressure is determined based onthe master cylinder pressure P_(M) as an operating force relating amountwhich relates to an operating force applied to the brake pedal 10 and arate of increase in the master cylinder pressure P_(M) as a rate ofincrease of the operating force relating amount.

The necessary negative pressure is set to a greater value when themaster cylinder pressure P_(M) is large than when the master cylinderpressure P_(M) is small. This is because the boost limit of the booster12 must be increased more when the master cylinder pressure P_(M) islarge than when the master cylinder pressure P_(M) is small since thebooster 12 is closer to its boost limit when the master cylinderpressure P_(M) is large than when the master cylinder pressure P_(M) issmall. If the necessary negative pressure is increased and the negativepressure in the negative pressure chamber 12 a is increased, the boostlimit of the booster 12 is increased, which condition allows boosting anoperating force by the same ratio.

The necessary negative pressure is set larger when a rate of increase inthe master cylinder pressure P_(M) is large than when the rate ofincrease in the master cylinder pressure P_(M) is small. This is becausethe negative pressure must be increased when the rate of increase islarge so as to maintain a sufficient operating speed of the booster 12.Additionally, a possibility for reaching the boost limit is higher whenthe rate of increase is large than when the rate of increase is small.If the master cylinder pressure P_(M) is equal, the possibility forreaching the boost limit is high when the rate of increase is largesince an operating force is rapidly increased. On the other hand, whenthe rate of increase is small, the possibility for reaching the boostlimit is low. If the rate of increase is a negative value and when theoperating force is being released, the possibility for reaching theboost limit is further decreased.

In the present embodiment, the negative pressure in the negativepressure chamber 12 a is not directly compared with the necessarynegative pressure but a comparison is made between the boost limit and avalue (hereinafter referred to as an operational state value) obtainedby adding a boost-limit-arrival estimating value (hereinafter referredto as an arrival estimating value) corresponding to the rate of increaseto the master cylinder pressure P_(M). If the degree of opening of thethrottle valve 20 is controlled so that a state in which the boost limitis larger than the operational state value is maintained, the negativepressure in the negative pressure chamber 12 a is maintained to begreater than the necessary negative pressure. As mentioned above, avalue of the master cylinder pressure P_(M) corresponds to the operatingforce applied to the brake pedal 10; the arrival estimating valuecorresponds to the rate of increase in the master cylinder pressureP_(M); and a value of the boost limit corresponds to the negativepressure in the negative pressure chamber 12 a. Accordingly, acomparison between the negative pressure in the negative pressurechamber 12 a and the necessary negative pressure corresponds to acomparison between the operational state value and the boost limit. Thearrival estimating value is set to a value α1 when the rate of increasein the master cylinder pressure P_(M) is a positive value, and is set toa value α2 which is smaller than the value α1 when the rate of increaseis a negative value. Since the possibility for rapidly reaching itsboost limit is higher when the rate of increase is large than when therate of increase is small, the arrival estimating value is increasedwhen the rate of increase is large. The operational state value is largewhen the operating-force relating amount is large and the rate ofincrease in the operating force is large.

When the boost limit is greater than the operational state value, thebooster 12 has not reached its boost limit, and, thus, it can bedetermined that the possibility for reaching the boost limit is low. Insuch a case, the negative pressure is greater than the necessarynegative pressure, and the degree of opening of the throttle valve 20 isnot required to be decreased. When the boost limit is smaller than thenecessary negative pressure, it can be determined that the boost limithas been reached or the possibility for rapidly reaching the boost limitis high. In such a case, the negative pressure is smaller than thenecessary negative pressure, and the degree of opening of the throttlevalve 20 is required to be decreased. Thus, the opening-degreedecreasing information is output to the engine control apparatus 220.

As mentioned above, if the negative pressure in the negative pressurechamber 12 a is maintained to be greater than the necessary negativepressure, the boost limit of the booster 12 can be increased so as toincrease a braking force even if an abnormality occurs in thebraking-force assisting apparatus 8.

In the present embodiment, the degree of opening of the throttle valve20 is decreased during the non-operating time, so that during theoperating time the negative pressure in the negative pressure chamber 12a is not decreased below the predetermined negative pressure which isdetermined to be the greater one of the necessary negative pressuredetermined as mentioned above and the operating-time preset negativepressure β2. That is, when the necessary negative pressure is smallerthan the operating-time preset negative pressure β2, the negativepressure in the negative pressure chamber 12 a is set so as not to besmaller than the operating-time preset negative pressure β2. On theother hand, when the necessary negative pressure is greater than theoperating-time preset negative pressure β2, the negative pressure in thenegative pressure chamber 12 a is set so as not to be smaller than thenecessary negative pressure.

A description will now be given, with reference to FIG. 8, of athrottle-opening-degree control operation.

First, in S11, it is determined whether or not the brake pedal 10 isbeing operated based on an output signal of the brake switch 150. If thebrake pedal 10 is being operated, the routine proceeds to step S12. Instep S12, it is determined whether or not there is an abnormality in thebraking-force assisting apparatus 8 by the abnormality detecting device158. If it is determined that there is an abnormality in thebraking-force assisting apparatus 8, the routine proceeds to step S13.In step S13, it is determined whether or not a negative pressure in thenegative pressure chamber 12 a is smaller than the operating-time presetnegative pressure β2. The negative pressure in the negative pressurechamber 12 a is known by the negative pressure information provided fromthe engine control apparatus 220. If the negative pressure is largerthan the operating-time preset negative pressure β2, that is, if thedetermination is negative, the routine proceeds to step S14. In stepS14, it is determined whether or not the negative pressure in thenegative pressure chamber 12 a is greater than the necessary negativepressure corresponding to the operating force applied to the brake pedal10. If the negative pressure is smaller than the necessary negativepressure, the routine proceeds to step S15 so as to output theopening-degree decreasing information. On the other hand, if thenegative pressure is greater than the necessary negative pressure, theroutine proceeds to step S16 so as to output the opening degree controlpermitting information.

If it is determined, in step S13, that the negative pressure in thenegative pressure chamber 12 a is smaller than the operating-time presetnegative pressure β2, that is, the determination in step S13 isaffirmative, the routine proceeds to step S15 so as to output theopening-degree decreasing information. Additionally, if it isdetermined, in step S12, that the braking-force assisting apparatus 8 isnormal, the routine proceeds to step S16. That is, if the braking-forceassisting apparatus 8 is normal, the opening-degree decreasinginformation is not output. In this case, a braking force is increased bythe braking-force assisting apparatus 8 after the booster 12 reaches itsboost limit as mentioned above.

Additionally, if it is determined, in step S11, that the brake pedal isnot being operated, the routine proceeds to step S17. In step S17, it isdetermined whether or not the negative pressure is smaller than thenon-operating-time preset negative pressure β1. If the negative pressureis smaller than the non-operating-time preset negative pressure β1, theroutine proceeds to step S15 so as to output the opening-degreedecreasing information. Otherwise, the routine proceeds to step S16.

A description will now be given, with reference to FIG. 9, of thedetermination in step S14 as to whether or not the negative pressure inthe negative pressure chamber 12 a is greater than the necessarynegative pressure.

First, in step S41, the boost limit P_(S) is obtained based on thenegative pressure in the negative pressure chamber 12 a, the informationof the negative pressure being provided by the engine control apparatus220. The boost limit P_(S) can be obtained according to a relationshipbetween the boost limit P_(S) and the negative pressure which is storedas table information shown in FIG. 4. In step S42, it is determinedwhether or not a rate of increase in the master cylinder pressure P_(M)is equal to or greater than zero. If the rate of increase is equal to orgreater than zero, the routine proceeds to step S43. In step S43, thearrival estimating value α is set to the value α1. On the other hand, ifthe rate of increase is less than zero, the routine proceeds to step S44so as to set the arrival estimating value α to the value α2. Then, instep S45, the boost limit P_(S) is compared with the operation statevalue which is obtained by adding the arrival estimating value a to themaster cylinder pressure P_(M).

If the boost limit P_(S) is smaller than the operation state value, theroutine proceeds to step S15 of FIG. 8 so as to output theopening-degree decreasing information. On the other hand, if the boostlimit P_(S) is equal to or greater than the operational state value, theroutine proceeds to step S16 of FIG. 8 so as to output theopening-degree control permitting information.

A description will now be given, with reference to FIG. 11, of aspecific example of the above-mentioned control operation. When thebrake pedal 10 is not being operated and when the negative pressure inthe negative pressure chamber 12 a is greater than thenon-operating-time preset negative pressure β1, the opening-degreedecreasing information is not output. However, when the negativepressure is decreased to a value smaller than the non-operating-timepreset negative pressure β1, the opening-degree decreasing informationis output. When the brake pedal 10 is not being operated, the negativepressure is maintained to a value greater than the non-operating timepreset negative value β1.

If the brake pedal 10 is operated at a time t1, the negative pressure isdecreased. However, if the negative pressure is maintained to be greaterthan the non-operating-time preset negative pressure β1 during thenon-operating time, the booster 12 rarely reaches its boost limit when anormal braking operation is performed. Accordingly, the operating forceapplied to the brake pedal 10 can be boosted with a constant ratio. Whenthe brake pedal 10 is being operated, the negative pressure iscontrolled so that the negative pressure is not smaller than theoperating-time preset negative pressure β2. However, if the necessarynegative pressure is greater than the operating-time preset negativepressure β2, the negative pressure is controlled so as not to be smallerthan the necessary negative pressure.

When the brake pedal 10 is operated along a single dashed chain line M,the operational state value is changed along a single dashed chain lineM′. In a range where the operating force is not very large, theoperational state value M′ cannot be greater than the operating-timepreset negative pressure β2. Accordingly, the degree of opening of thethrottle valve 20 is controlled so that the negative pressure in thenegative pressure chamber 12 a is not decreased to a value smaller thanthe operating-time preset negative pressure β2.

When the brake pedal 10 is operated with a large operating force, thatis, when the brake pedal 10 is operated along a double dashed chain lineN, the necessary negative pressure exceeds the operating-time presetnegative pressure β2 at a time t2. Accordingly, the negative pressure isincreased so that the boost limit is not decreased to a value smallerthan the operational state value N′.

As mentioned above, if the negative pressure in the negative pressurechamber 12 a is maintained to be greater than the necessary negativepressure, the boost limit can be increased in response to the operatingforce relating amount. Accordingly, if the brake pedal 10 is operatedwith a large operating force, the operating force can be appropriatelyboosted by the booster 12. Since the negative pressure in the negativepressure chamber 12 a cannot be an absolute vacuum, the boost limitcannot be a infinitely large value. However, if an abnormality occurs inthe braking-force assisting apparatus 8, the boost limit can beincreased so that the master cylinder pressure can be increased toincrease the braking force. In the present embodiment, when the brakingoperation is being performed, the opening-degree decreasing informationis output only when it is necessary. That is, the opening-degreedecreasing information is output only when an abnormality occurs in thebraking-force assisting apparatus 8 and when the negative pressure inthe negative pressure chamber 12 a is greater than one of theoperating-time preset negative pressure β2 and the necessary negativepressure, which one is greater than the other. Accordingly, the degreeof opening of the throttle valve 20 is positively decreased when it isnecessary, and is not decreased unnecessarily. Thus, a frequency ofdecreasing operations for decreasing the degree of opening of thethrottle valve 20 can be decreased so that deterioration in a rate offuel consumption due to a decrease in the degree of opening of thethrottle valve 20 when the ultra lean burn is performed can besuppressed. Additionally, since a change in the negative pressurecorresponding to the operating force when the braking operation is beingperformed, deterioration in a brake feel can be suppressed.

It should be noted that, in the above-mentioned embodiment, although thenegative pressure is set not to be smaller than one of theoperating-time preset negative pressure β2 and the necessary negativepressure, which one is greater than the other, the operating-time presetnegative pressure β2 is not necessarily considered and the negativepressure may be merely maintained so as not to be smaller than thenecessary negative pressure as shown in the flowchart of FIG. 12. Insuch a case, the boost limit corresponding to the negative pressure iscontrolled not to be smaller than the operating force relating amount M′or N′. In this case, it can be considered that the operating-time presetnegative pressure β2 corresponds to the necessary negative pressure (avariable value). However, in this case, the non-operating-time presetnegative pressure β1 must be set to a value which is not exceeded by theoperating-time preset negative pressure β2. That is, thenon-operating-time preset negative pressure β1 must be set to a valuegreater than a value obtained by adding an amount of decrease in thenegative pressure when a braking operation is performed to a value atwhich the boost limit is not reached when the brake pedal 10 is beingoperated with a large operating force. Alternatively, the necessarynegative pressure may not be considered and the negative pressure may becontrolled so as not to be smaller than the operating-time presetnegative pressure β2. Additionally, the negative pressure may becontrolled so as not to be smaller than one of the necessary negativepressure and the operating-time preset negative pressure β2, which oneis smaller than the other. In any case, a frequency of decreasingoperations for decreasing the degree of opening of the throttle valve 20can be decreased, and deterioration in a brake feel can be suppressed.

Additionally, the necessary negative pressure may be determined based ononly the master cylinder pressure P_(M) or only a rate of increase inthe master cylinder pressure P_(M), or may be determined by consideringother conditions. That is, the operational state value may be set to avalue of the master cylinder pressure P_(M) itself. Additionally, thearrival estimating value a may be continuously changed based on a rateof increase in the master cylinder pressure P_(M).

Further, values of the non-operating-time preset negative pressure β1and the operating-time preset negative pressure β2 are not limited tothe values described above, and other values may be used. For example,the operating-time preset negative pressure β2 may be set to a value atwhich the boost limit is not reached when the brake pedal is operatedwith a large operating force, and the non-operating-time preset negativepressure β1 is set to be a value greater than a value obtained by addingan amount of decrease in the negative pressure when the brake pedal 10is operated with a large operating force one time to the operating-timepreset negative pressure β2. Additionally, the operating-time presetnegative pressure β2 may be set to a minimum value which is needed forthe negative pressure chamber 12 a.

Additionally, the negative pressure may not necessarily be maintained tobe greater than the non-operating-time preset negative pressure β1 whena braking operation is not performed. In practice, the degree of openingof the throttle valve 20 may be decreased when the negative pressure isneeded, that is, only when the negative pressure in the negativepressure chamber 12 a becomes smaller than the necessary negativepressure. Additionally, the degree of opening of the throttle valve 20may be decreased only when a braking operation is not being performed.As mentioned above, if the negative pressure in the negative pressurechamber 12 a is sufficiently increased when a braking operation is notbeing performed, there may be a case in which it is not necessary todecrease the degree of opening of the throttle valve 20 when a brakingoperation is being performed.

Additionally, in the above-mentioned embodiment, when an abnormalityoccurs in the braking force assisting apparatus 8, it is determinedwhether or not the opening-degree decreasing condition is satisfied sothat the opening-degree decreasing information is output when theopening-degree decrease condition is satisfied. However, when anabnormality occurs in the braking-force assisting apparatus 8, thedegree of opening of the throttle valve 20 may be decreased by a valuecorresponding to a target boost limit (corresponding to a targetnegative pressure of the negative pressure chamber 12 a). The targetboost limit can be determined based on at least one of the mastercylinder pressure and a rate of increase in the master cylinderpressure. For example, according to a graph shown in FIG. 14, a degreeof opening may be increased when the target boost limit is small, andthe degree of opening may be decreased when the target boost limit islarge. In the present embodiment, the degree of opening of the throttlevalve 20 is controlled according to execution of athrottle-opening-degree control program represented by the flowchart ofFIG. 13.

In FIG. 13, it is determined, in step S61, whether or not a brakingoperation is being performed. If a braking operation is being performed,the routine proceeds to step S62. In step S62, it is determined whetheror not an abnormality occurs in the braking force assisting apparatus 8.If it is determined that an abnormality occurs in the braking-forceassisting apparatus 8, the routine proceeds to step S63. In step S63,the degree of opening of the throttle valve 20 is determined accordingto the target negative pressure, and information for instructing adecrease of the degree of opening of the throttle valve 20 to thedetermined degree of opening is output. In such a case, opening-degreeinformation which represents the degree of opening is output togetherwith or instead of the opening-degree decreasing information.Additionally, an opening-degree decreasing time which represents aperiod for decreasing the degree of opening may be obtained based on thetarget boost limit so that opening-degree decreasing time information isoutput.

Additionally, the negative pressure may be controlled irrespective ofwhether the braking-force assisting apparatus 8 is normal or abnormal sothat the negative pressure does not become smaller than the necessarynegative pressure. If the negative pressure is controlled so as not tobe smaller than the necessary negative pressure, an appropriate boostcan be achieved if a braking force is large which results in a brakingforce corresponding to an operating force.

If the boost limit is increased when the braking force assistingapparatus 8 is normal, an operation of the brake force assistingapparatus 8 can be delayed. This provides an advantage in that a drivingnoise of the pump 40 is reduced.

Additionally, the braking-force assisting apparatus 8 may be operatedwhen an emergency braking operation is required instead of beingoperated after the booster limit is reached. In such a case, it isdetermined whether or not a large braking force is required based on anoperational speed of the brake pedal 10 so that the braking-forceassisting apparatus 8 is operated so as to increase a braking force whenit is determined that the a large braking force is required. In such acase, it is preferable that the negative pressure be sufficientlyincreased even when a braking operation is not being performed.Additionally, the braking force assisting apparatus 8 is not limited tothe above-mentioned structure, and other structures may be used. Forexample, the brake-force assisting apparatus 8 may include anaccumulator. Additionally, the pressure control valve 30 of the brakingforce assisting apparatus may be replaced with a simple solenoid valve.A pressure in the brake cylinders 26 may be controlled by operations ofthe pressure-increasing valves 90 and the pressure-decreasing valves100, or may be controlled by an operation of the pump motor 160.Further, the braking-force assisting apparatus 8 is not essential. Thatis, the boost limit can be increased by increasing the negative pressurewithout using the braking-force assisting apparatus 8.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority applicationNo.10-105119 filed on Apr. 15, 1998, the entire contents of which arehereby incorporated by reference.

What is claimed is:
 1. A vacuum booster apparatus adapted to be used fora brake apparatus provided in a vehicle having an internal combustionengine, said vacuum booster apparatus comprising: a vacuum boosterhaving a first pressure chamber and a second pressure chamber, saidfirst pressure chamber being connected to an intake pipe of said engineon a downstream side of an intake passage opening and closing valve,said second pressure chamber being selectively connectable to one ofsaid first pressure chamber and atmosphere, said vacuum booster boostingan operating force applied to a brake opening member based on a pressuredifference between said first pressure chamber and said second pressurechamber; and opening-degree control device controlling a negativepressure in said first pressure chamber by controlling a degree ofopening of said intake passage opening and closing valve, wherein saidopening-degree control device includes limited-time opening-degreedecreasing means for decreasing the degree of opening of said intakepassage opening and closing valve only for a limited time when thenegative pressure in said first pressure chamber is smaller than apredetermined necessary negative pressure that is varied based upon anoperating-force relating amount related to the operating force appliedto said brake operating member.
 2. The vacuum booster apparatus asclaimed in claim 1, wherein said limited-time opening-degree decreasingmeans includes necessary-negative-pressure determining means fordetermining said predetermined necessary negative pressure in accordancewith the operating force relating amount related to the operating forceapplied to said brake operating member.
 3. The vacuum booster apparatusas claimed in claim 1, wherein said limited-time opening-degreedecreasing means includes means for determining said predeterminednecessary negative pressure in accordance with a rate of increase in anoperating-force relating amount related to the operating force appliedto said brake operating member.
 4. The vacuum booster apparatus asclaimed in claim 1, wherein the limited-time opening-degree decreasingmeans decreases the degree of opening of said intake passage opening andclosing valve only when a boost limit of said vacuum booster is smallerthan an operational state value which is determined according to anoperating-force relating amount related to the operating force appliedto said brake operating member.
 5. A vacuum booster apparatus adapted tobe used for a brake apparatus provided in a vehicle having an internalcombustion engine, said vacuum booster apparatus comprising: a vacuumbooster having a first pressure chamber and a second pressure chamber,said first pressure chamber being connected to an intake pipe of saidengine on a downstream side of an intake passage opening and closingvalve, said second pressure chamber being selectively connectable to oneof said first pressure chamber and atmosphere, said vacuum boosterboosting an operating force applied to a brake operating member based ona pressure difference between said first pressure chamber and saidsecond pressure chamber; and an opening-degree control devicecontrolling a degree of opening of said intake passage opening andclosing valve in accordance with a negative pressure in said firstpressure chamber and a rate of increase in an operating-force relatingamount, the operating-force relating amount related to an operatingforce applied to said brake operating member.